1. Field of the Invention
This invention relates to a zinc alkaline secondary battery such as the nickel-zinc secondary battery or silver-zinc secondary battery in which zinc is used as the negative active material. More particularly, it relates to a zinc alkaline secondary battery having an improved service life in which a multi-layer separator is set between the positive electrode and the negative electrode and a quantity of an alkaline electrolyte to be absorbed and retained in the layer of the separator adjacent to the negative electrode is less than that in the layer of the separator adjacent to the positive electrode.
2. Description of the Prior Art
In recent years, remarkable dissemination of portable electronic and electric devices has come to accelerate rapid replacement of primary dry cells by secondary batteries, especially nickel-cadmium secondary batteries. As a more capable version of such secondary batteries, the zinc alkaline secondary battery are drawing keen attention. The zinc alkaline secondary battery enjoys various advantages such as high energy density per weight, high and stable operating voltages, good low-temperature requirements and superiority on economy and safety, but shows a disadvantage that its service life, namely the charge-discharge cycle life is short.
This disadvantage is believed to be caused basically by the reason that zinc on the surface of the negative electrode is dissolved as zincate ion into the electrolyte during the discharge. While, during the charge, this zincate ion forms dendritic zinc or readily shedding mossy zinc on the surface of the negative electrode, which results in short circuit between the negative electrode and the positive electrode or deformation of the negative electrode and consequently in a lowering of the service life of the battery. Further, the formation of the zincate ion during the discharge lowers a concentration of zinc in the surface part of the negative electrode, which results in a difference between zinc concentration of the surface part and that of the inner part, and an increase of a binder concentration and a lowering of the electroconductivity or the electrolyte content at the surface part.
It is known to use a limited quantity of electrolyte because the formation of zincate ion is accelerated in the presence of an excess quantity of electrolyte (21st Power Source Symposium, 1967, p 70-73). Also, it is known to use a first layer adjacent to the negative zinc electrode which is more retentive of electrolyte than in a second layer adjacent to the positive electrode (U.S. Pat. No. 3,669,746).
Under such circumstances, the inventors of this invention have studied to overcome the above disadvantages of the zinc alkaline secondary battery and found that the cycle life can be improved when a quantity of electrolyte is limited to the extent sufficient to accomodate electrochemical conduction between the electrodes and does not have substantially free electrolyte in the battery case, and a quantity of the electrolyte absorbed and retained by the layer of the separator adjacent to the zinc electrode is less than that of the positive electrode.